Many manufacturing processes electronic for electronic components and assemblies include inspection and test procedures, which can be either manual or automated. For example, the surface mount assembly process (SMT) consists fundamentally of three value added process steps: Solder paste printing, component mounting and reflow. These are schematically illustrated in FIG. 1. Un-stack bare board 30 removes a single bare circuit board from a stack of them and inserts it into the assembly line. Solder paste print 32 prints solder paste onto the bare circuit board. Component mount 34 moves components from a component bulk feed apparatus (not shown) and places them onto the circuit board. Reflow oven 36 melts the solder paste and then allows it to cool and re-solidify. Stack populated board 38 takes at least partially assembled circuit boards and stacks them into an easily portable batch.
There are many extensions and variations to the above, including inspection and test strategies, flipping the circuit boards so that components can be mounted onto each side, accommodation for through-hole components, glue deposition for those components that need to be held during assembly, odd-form assembly, under fill of flip-chip components, etc. But at its most basic level, the above steps describe the SMT line.
While the SMT assembly process has been refined, SMT assembly lines continue to make a variety of errors that lead to defective assemblies. These errors are herein grouped into classes:
One class of errors is related to solder paste printing and includes (non-exhaustive list): Incorrect height, volume or area of each individual solder paste deposit; Incorrect position (sometimes called solder paste registration errors); and Creation of a solder bridge (paste connecting two nominally separate deposits).
Another class of errors is related to components and includes (non-exhaustive list): Missing components (one or more not located where they should be); Extra components (one or more located where they shouldn't be); Incorrect components (wrong component type or value); and Incorrectly positioned components (linear or rotational errors).
Yet another class of errors comes from the reflow oven (non-exhaustive list): Incorrect temperatures; Incorrect dwell time in temperature zones; and Uneven heating.
Yet another class of error comes from raw materials imperfections (non-exhaustive list): Component lead oxidation; Component lead non-coplanarity; Panel (circuit board) surface contamination; Panel warp; Panel stretch (relative to the solder paste stencil); Stencil stretch (relative to the panel); Stencil apertures incorrectly sized, shaped, positioned; and Insufficient solvents in the paste.
Yet another class of error comes from design imperfections (non-exhaustive list): Components are placed too closely together or otherwise positioned so that, when the panel is in the oven, uneven heating causes solder paste to melt in a non-uniform way; and Copper pads and/or solder mask are incorrectly sized/positioned causing incorrect solder wicking.
Most of these errors have some visible manifestation when viewed after solder reflow. These manifestations include (non-exhaustive list): Missing components (compared to the design intent); Extra components (compared to the design intent); Wrong component (as compared to the design intent); Wrong value (e.g., the resistance or capacitance of a component is not correct, even though the package is right. This is a subset of wrong component); Tombstone (a two lead component flipped up on end so that it is making contact only on one lead); Billboard (a two lead component flipped up on its side so that it is making contact on both leads, but not with a correct solder fillet); Dead Bug (a component flipped upside down “feet up”); Wrong polarity (a device whose orientation must be controlled for correct electrical behavior but is oriented incorrectly); Bad solder joint (one or more solder joints is improperly formed. Solder joints that are located under the body of a component would not normally be visible but can be observed via x-ray inspection); Lifted lead (not all the leads of a component are soldered well to the panel. Caused by component coplanarity errors, panel warp or both); and Solder bridge (two leads that should be electrically isolated have electrical continuity).
Defects can result in electronic assemblies that do not work correctly while they are still in the factory. It is possible to catch most of these before the assembly is shipped by electrically testing the completed assembly, depending on the thoroughness of the test. Thorough electrical testing is often quite difficult and time consuming, especially for more complex electrical assemblies. For some devices, thorough electrical test is so difficult as to be considered impractical.
Sometimes, assemblies will work properly when electrically tested at the factory and then fail after only a short time in the field. These failures are often caused by visually evident errors. For example, a partially formed solder joint will provide good electrical contact but possibly tenuous mechanical contact. Thermally or mechanically induced physical stresses can cause this solder joint to crack leading to premature field failure. However, improperly formed solder joints can be visually detected if they are not hidden from view (e.g. by a component body or a radio frequency (RF) shield, or the like).
Accordingly, electrical test is generally understood to be an incomplete quality control approach. To supplement electrical test, SMT operators nearly always implement some sort of visual inspection approach at the end of the assembly line (after reflow). One type of inspection is by human visual. Another, often more suitable approach, is in-line AOI (Automatic Optical Inspection) and sometimes X-Ray (automatic or manual) inspection.
AOI machines are nearly always two dimensional (2D); that is they acquire (most often) color images of the assembled panel and analyze the luminance and chrominance characteristics of those images to determine if the appearance of the panel is acceptable according to criteria established through the programming efforts described above.
Three dimensional (3D) AOI machines, while uncommon, are known. 3D AOI machines offer the advantage of detection of the relevant attributes of a component such as its presence and position at least in part based upon its height, rather than solely upon its luminance or color. The key advantage of this method is that all components, because of their basic mechanical nature, will stand up above a substantially planar substrate or panel. As stated above, 2D AOI machines must be sensitive to the component's appearance, which can change from vendor to vendor and, at the extreme, could be the same as the panel thereby making it invisible to a 2D sensing methodology.
3D AOI has not generally been adopted in the SMT industry because it is complicated, slow (the scan time is long) and expensive compared to 2D AOI. Also, a height map of a component on a panel is insufficient to determine all the interesting characteristics of a panel, so a system that only acquired 3D data would be insufficient to screen for all of the possible errors detectable after SMT reflow.
There are many extant methods for acquiring height data on the scale required for 3D AOI. Among them are: Laser triangulation; Shadow casting; and Phase Profilometry.
The best method will depend upon the target's optical and mechanical characteristics, the requirements for speed and cost and the nature of which exact measurements are needed.
Laser triangulation is widely used and can be accomplished by projecting a spot or a line or a series of lines onto the target. Spot projection, see FIG. 2, has the advantage of wide dynamic range, but is very slow compared to line projection techniques. In FIG. 2, which is a simplified schematic side view, a spot projector 21 generates directed light 22 to illuminate a target 18 situated on a substrate 10 with an optional substrate top coating 11. In the illustrated configuration, light from the spot 23 striking the top of the component 18 is diffusely scattered and some of this scattered light is collected by light receiver 24. Light scattered from spot 23 appears to camera 24 to shift laterally depending on the height 19 of the scattering surface 18. Note the coordinate system 20 wherein “height” is meant to be substantially parallel to the Z direction.
Referring to FIG. 3, which is a top view and is schematically what the light receiver 24 of FIG. 2 sees as it looks down on the scene. Referring to coordinate system 20, spot 5 has a certain position in the X direction in the image. If the object with height 18 were missing from the scene, the directed light 22 would strike the substrate top coating 11 and the spot would appear to scatter light from position 6. The apparent lateral X displacement between position 5 and position 6 is thus an indication of the height of the top surface of object 18 above substrate top coating 11.
While they can be very accurate, simple spot range measurement techniques are slow, because the height is measured from only one spot a time. A spot scanning mechanism such as a moving mirror, an Acousto-Optic Deflector (AO cell) or the like is often used to speed this up but these approaches add substantial complexity and cost. High speed spot projectors are implemented with high power lasers in order to generate the required signal within a short time. These require safety precautions according to rules established by the US FDA.
Line scanners can be faster than spot projectors but suffer from multi-path (reflections from nearby targets) and do not have as wide a dynamic range. However, they are typically cheaper and simpler than spot based laser triangulation systems with scanners and may possibly be operated at lower power levels than spot projectors.
FIG. 4 illustrates how the scanning spot and line projector systems work. The light projector 21 directs a flying spot or a sheet of light 22 onto feature with height 18 above substrate 10. The camera, omitted from this drawing for clarity, will observe light scattered from illuminated line segments 5 and 6. Line segment 5 is returned to the camera from the top of the feature with height 18. Line segments 6 are returned from the substrate 10. The lateral X displacement of points along line segment 5 from points along line segments 6 is a measure of the heights of those points relative to one another. Distinct from the spot scanner of FIGS. 2 and 3, in this case, a plurality of height measurements is available because data from multiple points on each line have been acquired.
For a given cost or complexity, phase profilometry is the fastest known method because a potentially large 2D area of the target can be illuminated at once (as compared to a spot or a line). Phase profilometry is also most often implemented with white light from a strobe or other light source and is therefore not subject to FDA laser safety considerations.
The technique of phase profilometry is widely used for forming height maps of substantially diffuse targets. See for example U.S. Pat. No. 4,641,972 (incorporated by reference). This technique is also used for forming height maps of highly specular targets such as mirrors. For the former use, the illuminator is at a non-specular angle from the image acquisition optics (as was the case for the spot range finder of FIG. 2). For the later use, the illuminator and the acquisition optics are arranged to be substantially at the specular angle (not illustrated).
A significant component in a phase profilometry system is a structured light projector, shown schematically at 21, 30, 31 and 32 in FIG. 5, which replaces the spot or line projectors of above. This projector differs from the line projector in that, instead of projecting light along a thin sheet that when striking the target surface makes a contour, this projector projects intensity modulated light 22 along a volume that when striking the target surface, illuminates a two dimensional area on that surface with that intensity pattern.
In FIG. 5, light 22 striking the target surface of object 18 is scattered diffusely as before and again, some of it is captured by camera 24. The scattered light that enters the camera is illustrated in this drawing as ray 36 but it should be construed to a volume of light that will form a 2D image within camera 24.
Like the line and spot projectors of FIGS. 2 through 4, the light is projected along a first direction. Camera 24 observes the scene from a second or observation direction. These two directions are not parallel and the included angle 34 between them is called the triangulation angle. In FIG. 5, the observation direction is substantially perpendicular to the substrate surface 10. A pattern is superimposed up the projected light such that the pattern, or portions thereof, will appear, when viewed by the camera 24, to shift laterally as it strikes objects of varying height. This lateral shift, which in a repetitive pattern can be considered a phase shift, is indicative of the heights of an area of points on the surface in the same general way as the lateral shift of the spot or the line is so indicative of a single point or a linear grouping of points.
The advantage of the line projector over the spot projector is that height data of a plurality of points along the illuminated contour can be acquired instead of only at the one illuminated spot.
Phase profilometry has an advantage over the line projection technique in that height data of a plurality of points in the illuminated area can be acquired instead of only along the one illuminated line.
Referring still to FIG. 5, light from light projector 21 passes through reticle or grating 30 and is directed onto target 18 and top surface of substrate 10 with optional coating 11. Light scattered from target 18 is captured by receive optical system 33 and passed to camera 24 for later processing by electronics (not shown).
Although FIG. 5 is a 2D drawing, it will be understood that the light 22 from projection system 21, 30, 31, and 32 is illuminating an area of the top surface of target 18 and top surface of substrate 10 or coating 11.
There are numerous ways to introduce the pattern to the projected light (non-exhaustive list):                1. Projection of light through a square wave grating or ruling generating square wave patterns on the target.        2. Projection of light through a pixilated grating allowing for generation of sinusoidal patterns. Usually some sort of spatial low pass filter is employed to suppress the pixelization leaving only the low frequency sinusoid.        3. Defocusing of the above to suppress harmonics (in the case of -1-) or the individual pixels (in the case of -2-)        4. Astigmatic projection of a sinusoidal pattern to generate a sinusoidal pattern.        
The classic characteristics of the projector for SMT inspection are:                Sinusoid projection pattern.        Telecentric optics providing substantially constant magnification over changing distance from the projector to the target surface.        Scheimpflug condition optics for the projector: Referring to again FIG. 5, the projected patterned light beam 22 is at an angle 34 from the normal to the top substrate surface 10 and 11. A projection system that conforms to the Scheimpflug condition allows the projected pattern's focal plane to be parallel to the target surface even when the optical axis is off normal. Satisfaction of the Scheimpflug condition requires the reticle or grating 30 to be rotated from the optical axis of the projector.        
Classically, three images are acquired of substantially the same field of view. The three images are such that the phase of the projected pattern is changed for each of them; usually the phases are 0, 120 and 240 degrees. Other combinations of phases can work and are sometimes used for mechanical convenience. Three images are the minimum required to unambiguously resolve the three inherent ambiguities of the target which are:                1. Brightness        2. Vector Phase        3. Vector Length        
The Brightness refers to how bright a region of the target is as measured by the amount of structured light observed returning from that region to the observation camera.
The Vector Phase refers to the observed phase of the projected pattern as modified (shifted laterally) by height features on the target. When an idealized projector projects the pattern onto a flat planar surface devoid of height features, the Vector Phase will change according to the projection frequency only. In the presence of height variations, the phase will vary from the above in relation to those height variations.
The Vector Length refers to the fraction of the projected modulation that is returned from a region on the target to the camera. It can be used to determine the reliability or quality of the measurement of the Vector Phase; the smaller the Vector Length, the noisier the Vector Phase.
All three of these unknowns can be unambiguously solved by the application of public domain phase reconstruction algorithms to the three images taken at 120° phase shift from each other, or to four images taken at 90° phase shift from each other, or to any n images, n≧3, where the phase shift between the images is known, not zero, and not 360° or a multiple thereof.
Exemplary reconstruction equations and an approach for the three image reconstruction are disclosed in U.S. Pat. No. 6,750,899 B1, which is incorporated by reference. According to the '899 patent, a generalized approach allows us to compute H from images where the phase differences between successive images are known but unequal. The normalized intensity value for each pixel in the three-image co-sited set is given in Equation 1):
      (                            A                                      B                                      C                      )    =      r    ⁡          (                                                  1              +                              m                ⁢                                                                  ⁢                                  cos                  ⁡                                      (                                          ϕ                      -                                              ϕ                        a                                                              )                                                                                                                          1              +                              m                ⁢                                                                  ⁢                                  cos                  ⁡                                      (                                          ϕ                      -                                              ϕ                        b                                                              )                                                                                                                          1              +                              m                ⁢                                                                  ⁢                                  cos                  ⁡                                      (                                          ϕ                      -                                              ϕ                        c                                                              )                                                                                          )      
where r is the normalized reflectance at the pixel (the brightness), the known phase angles of the three fringes are φa, φb, φc and the relative phase φ of the fringe at the pixel is related to the projected fringe frequencies, pixel coordinate and z position by Equation 2):φ=2π(fxx+fyy+fzż)
To linearize the problem and make it more easily computed, the quantities are defined as in Equation 3):x=m cos φy=m sin φ
Then, Equation 1) can be re-written as in Equation 4):
      (                            A                                      B                                      C                      )    =            (                                    1                                              cos              ⁢                                                          ⁢                              ϕ                a                                                                        sin              ⁢                                                          ⁢                              ϕ                a                                                                          1                                              cos              ⁢                                                          ⁢                              ϕ                b                                                                        sin              ⁢                                                          ⁢                              ϕ                b                                                                          1                                              cos              ⁢                                                          ⁢                              ϕ                c                                                                        sin              ⁢                                                          ⁢                              ϕ                c                                                        )        ⁢          (                                    r                                                x                                                y                              )      
Through standard linear algebra, the system matrix in Equation 4) can be solved for r, x, and y. From x, y, the phase φ of the pixel can be computed by the processor in Equation 5):φ=tan−1(y/x)
Once the phase φ is computed in Equation 5), we multiply by an appropriate calibration scaling factor to compute the height of the pixel. Once all the heights for all the pixels are computed, the height map, H, is completed and ready for summary processing and display, as appropriate. An example height map is shown in FIG. 4A of the '899 patent.
Note that the above approach is only one of numerous formulations for arriving at the phase, and therefore the height map, from n≧3 phase shifted images.
As mentioned, the classic projection optical arrangement is telecentric. However, telecentricity is expensive and bulky to implement. Digitally correcting for non-telecentricity is known, but it is computationally intensive. Telecentricity has been chosen despite its drawbacks because it eliminates the compute burden required to correct the image for effects caused by variable range to the target. In the interests of the high throughput speeds required of in-line systems, this has been an appropriate tradeoff.
Off-line inspection systems, however, do not have the same high throughput speed requirements, so the extra cost and bulk of a telecentric projector is wasteful for that use.
There are numerous ways to generate images with the required phase shifts. One is to move an entire camera/projector assembly relative to the target. The phase pattern projected onto the target will shift accordingly. Re-registering the acquired image based on knowledge of the physical distance traversed will yield the required images. This method sacrifices a portion of the field of view but offers the advantage of opto-mechanical simplicity. Also, when coupled with a strobe lamp based illumination system, this method can provide the advantage of high speed. The motion system however must be very precise so as to allow re-registration to occur with the required precision; about one to two microns when used for solder paste inspection. Also, there are stringent demands placed upon the maximum allowable distortion of the optics in such a system. The required motion precision and high quality optics can make such a system expensive. There is at least one extant mechanism that operates this way.
Another approach is to keep the camera substantially stationary relative to the target and move the projector or an optical element within it so as to cause the projected fringe pattern to shift the desired amount. Referring again to FIG. 5, mechanical actuator 31 causes reticle or grating 30 to move a small distance between image acquisitions by camera 24. This small distance causes the projected pattern 22 to shift accordingly thereby introducing the required phase shift between image acquisitions. There are many electro-mechanical ways to do this including the use of moving mirrors or refractors. This motion must also be precise, or at least, precisely known, in order to be sure that the phases of the projected pattern have the right, or at least precisely known, phase shifts between them. Errors in this motion result in incorrect computation of the Vector Phase and therefore the heights. Incorrect height measurements can lead to False Calls (occurrences where the inspection device detects an error when, in fact, none is present) or False Accepts (occurrences where the inspection device determines that no error is present, when in fact one is). These motion systems can be costly, bulky, may have physical wear concerns leading to breakdowns or periodic service requirements and may be slow.
All the above mentioned mechanisms are incapable of removing the pattern from the projected light.
Though some of them, as illustrated in FIG. 5, are able to internally shift the phase of the projected pattern, none are capable of changing the spatial frequency of that pattern because it is substantially fixed by the optics 32 and the nature of the pattern of the reticle or grating 30.
One significant challenge relating to the use of phase profilometry to form height images of a circuit board is the mix of specular and diffuse features on the target surface. An example of a diffuse feature is the top a textured, typically grey component. An example of a specular feature is a solder joint or the top of a shiny component.
Shiny features that happen to be oriented so as to reflect light from the illumination system directly into the camera will appear to be very bright and will saturate the imager (e.g. CCD or CMOS area array or the like) located in the camera. The precise quantification of received light required to perform accurate phase profilometry will be lost for those pixels that are saturated. In the worst case, these very bright features can cause blooming on the imager, a phenomenon that will corrupt nearby pixels. At the other extreme, shiny features that are oriented so as to reflect light entirely away from the illumination system will appear to be very dark, so that again, the precise quantification of light required for accurate height calculations will be inhibited by various sources of noise (e.g. shot noise, dark current, quantification, etc.).
For this reason, forming a high fidelity height map of this mix of features from a single sensor system requires that system to have a very large dynamic range, preferably on the order of five decades. A large dynamic range allows bright reflections from specular surfaces to be imaged without saturation or blooming while also allowing data from dark areas to be acquired with an acceptable signal to noise ratio (SNR). Laser point range sensors can achieve this dynamic range at the cost of extremely slow throughput.
Techniques to extend the dynamic range of area based imagers are known and used in digital photography. Typically, images—of a scene with varying and precisely known exposure times are acquired, for example one under exposed, one properly exposed and one overexposed. These images are then merged according to some rule related to the exposure times and apparent brightness of the three images on a pixel by pixel basis. For example, saturated pixels in the overexposed image are not used. Instead, values for those pixels are used from either the properly exposed image or the under exposed image and then scaled according to the precisely known exposure time. The dynamic range of the resulting composite image can be orders of magnitude greater than that of any one single image, depending on the ratio of the exposure times.
Of course, this approach is not easily adaptable to moving scenes. US Patent Publication No. 2002191834 teaches a way to achieve this function with moving scenes using a strobe lamp.
Systems with only three orders of dynamic range are known to work well enough for SPI, because solder paste before reflow behaves, in aggregate, substantially like an optically diffuse surface. However, when components are added and especially once solder paste is reflowed; the specular or shiny conditions described above occur in abundance throughout the assembled circuit board.
One of the problems with phase profilometry is the ambiguity caused by 360° phase shifts. Slowly (spatially) shifting phase, where the spatial sampling density is in excess of the Nyquist limit, can be accommodated by phase unwrapping algorithms, but sudden changes are inherent in many target surfaces present in the SMT application, especially for components on panels. Phase unwrapping would be very unreliable in this application.
If the sudden changes are limited in size to substantially less than 180°, then the phase is readily computable without resorting to phase unwrapping. Solder paste deposits tend to be approximately 200 μm in height or less, so an appropriate phase wrap height to choose for this application is 500 μm or so, and under these conditions, phase unwrapping is unneeded.
However, for 3D AOI, the target surface can have sudden height changes on the order of 20 mm. For this application, a phase wrap height would preferably be 50 mm or so.
Referring to FIG. 6, the pattern projector 40 (considered to include everything required to project a pattern onto the target surface, including the light source, pattern introduction means and optics) illuminates an area 41 on the substrate surface 10 with optional coating 11. The illumination area 41 projects partially onto object with height 18. The pattern is an intensity modulated sinusoid with wave crests or troughs illustrated schematically by parallel lines 46 and 47.
Lines 46 are illuminating the top surface of the substrate 10 and can be used to compute the height of points on that surface. Lines 47 are illuminating the top surface of the object with height 18. The lateral shift 48 between points on these lines is a measure of the height difference between points on the substrate surface and points on the object with height surface.
The wave pattern has a wave direction 42. In this example, the wave direction 42 is not parallel to the projection azimuth angle direction (angle 44 to the X axis 20) and the included angle between the two is shown at 45.
Referring to FIGS. 5 and 6, increasing the wrap height (the height step that corresponds to a 360° phase shift in the repetitive projection pattern) can be done by increasing the wavelength of the projected pattern or decreasing the included angle 34 of FIG. 5 between the source and the receiver, or increasing the included angle 45 of FIG. 6.
However it is done, increasing the wrap height has the negative effect of decreasing the system's sensitivity to Z height changes, essentially reducing the system's resolution in that direction.
The preferred condition for an inspection system suited to both 3D SPI and 3D AOI is to have the high Z sensitivity concomitant to a short wrap height and have the phase determinism concomitant to a large wrap height.
As of this writing, all known phase profilometry approaches use a fixed wavelength phase projector with a fixed included angle 34 of FIG. 5 and with a fixed included angle 45 of FIG. 6 and are therefore unable to vary their wrap heights.
The use of three phase projections, where classically a phase shift of 120° is used there between to generate patterns from which a height map can be made, does not measure r directly. If r can be measured directly, and if multiple patterns, for example of varying wavelength, are used to extend the wrap height, then one r can be used to normalize two or more sets of x and y, thereby reducing the number of images that must be acquired.
Extant phase profilometry systems have fixed illumination azimuth angles 44 of FIG. 6. Most often, only one such angle is used. One system is able to use two fixed illumination azimuth angles wherein a single projector is used to illuminate a macroscopically moving mirror that directs the projected pattern to one or another of two physically distinct optical systems, comprised of mirrors, and deployed to project light either at illumination azimuth angle 44 of FIG. 6 or that angle+180°. The advantage that two illumination azimuth angles yields relates to shadows. Height objects with sharply rising sides may have a portion of their surface in shadow when only one illumination azimuth angle is used. In that case, information from the shadow region is unavailable to the inspection system. Thus a projection system able to use two illumination azimuth angles offset from one another by 180° has an increased probability of being able to acquire data from the shadow region at the expense of increased data acquisition time (time for the moving mirror to move and settle plus time the additional images to be acquired), data processing time and increased mechanical complexity (the moving mirror).
In the above mentioned system all the projection optics save the moving mirror are stationary. Thus the illumination azimuth angles are not variable but are fixed by the stationary optics and are selected for use, one at a time, by the macroscopically moving mirror.
The above approach works well for solder paste inspection where all the height features (solder paste) are at nearly the same height. Solder paste deposits, when printed properly, are spaced so that two illumination azimuth angles offset by 180° from each other will almost certainly allow for a view of the entire surface of each deposit.
However, for 3D AOI, where targets of interest with substantially different height may be situated adjacent to each other, no specific predetermined, fixed illumination azimuth angle or even pair of such angles can be assured of casting light onto the shorter target. This is especially true for solder joint inspection. Solder joints have heights at elevations at or very near to the top surface of the circuit board; i.e. they are very short targets.
In-line solder paste inspection (SPI) is often used to screen out incorrect solder paste prints before an erroneously printed panel can proceed down the assembly line. These inspection machines are available to inspect solder paste in 2D or 3D. Solder paste printers often are able to implement in-printer inspection (both 2D and 3D are available) but the time available for in-printer inspection is severely limited by the throughput requirements of the line. 3D is preferred because a significant portion of the solder paste printing errors are detectable only by devices that are sensitive to height and volume, not just area and XY position. However, 3D inspection machines tend to be substantially more expensive and somewhat slower than 2D.
In-line 3D SPI machines are quite expensive (˜US$100,000) and are sometimes difficult to program. They take up floor space in factories where this is sometimes at a premium. They cannot be used to service more than one line at a time.
In-line 2D SPI machines are less expensive (˜US$60,000) and are often more difficult to program than 3D.
One of the costly subsystems in both for SPI and AOI machines is the transport mechanism. This allows the projector/camera subsystem to tour large regions of the circuit board, regions that are larger than the camera's field of view. For in-line systems, these mechanisms must move quickly, as time is of the essence. Also, for many extant systems, they must move precisely, because imprecision in their movement causes imprecision in their measurements related to X and Y target positions (a lateral error, one in the XY plane, of the target as compared to the design intent). Examples of these measurements are the solder paste registration and a component position error.
Some in-line machines are claimed to implement both AOI and SPI thereby permitting their owners to move those machines to either inspection point as required. The extant dual-mode machines are able to implement 2D AOI inspection only. Of course, when it is in-line, the machine can implement only one of these functions at a time.
If operated in an off-line way such extant dual-mode machines can be switched from one mode to the other. But they require the user to manually change sensor heads; one head is able to perform SPI, the other AOI, and only one head can be situated within the machine at one time. So switching from one mode to the other requires a time consuming physical reconfiguration of the machine. Additionally, it may be necessary to run different mode-specific software applications.
A single off-line dual-purpose machine able to perform both 3D SPI and 3D AOI would be able to merge data acquired during SPI functions with data acquired during AOI functions applied to the same panel. There are many ways data can be profitably shared. One such way relates to programming or training the system to perform SPI and/or AOI. Once the system is programmed to perform, for example, SPI, much of what is needed to train the system to perform AOI is already known. Other ways of sharing data related to training are disclosed in the incorporated PCT and U.S. Provisional Application.
Another way data can be profitable shared can be seen by considering that a particular instance of a panel is inspected for SPI and later, when components have been mounted and solder reflowed, again the same panel is inspected for AOI. If a defect was found at AOI inspection, whether or not a corresponding defect was found at the same location during SPI inspection, it would be advantageous to present to the user all data and images from the SPI observation of the relevant location. These data and images, 2D or 3D, can be useful in determining the cause of the failure detected at AOI.
Although software and specialized systems exist for this purpose, in-line inspection machines are unable to perform this merging of data by themselves, because they are different machines located at different points in the SMT line. However, two separate machines of the same design, one at the SPI location and the other at the AOI location, can share image data in the same way as a single dual-purpose machine, situated off-line. Profitable sharing of image data is facilitated because the optics of both machines are nominally identical, so “difference” based image processing is greatly facilitated.
Also, a single off-line machine able to perform both 3D SPI and 3D AOI would cost less than two special purpose machines, occupy less work space on often crowded shop floors, would naturally have a single user interface for users to learn, would require fewer spare parts and in general, be simpler and cheaper to use and maintain.
For these and other reasons, there is a need for the present invention.